public final class Stringimplements java.io.Serializable, Comparable<String>, CharSequence
{/** The value is used for character storage. */private final char value[];/** The offset is the first index of the storage that is used. */private final int offset;/** The count is the number of characters in the String. */private final int count;/** Cache the hash code for the string */private int hash; // Default to 0/** use serialVersionUID from JDK 1.0.2 for interoperability */private static final long serialVersionUID = -6849794470754667710L;........
}

public String substring(int beginIndex, int endIndex) {if (beginIndex < 0) {throw new StringIndexOutOfBoundsException(beginIndex);}if (endIndex > count) {throw new StringIndexOutOfBoundsException(endIndex);}if (beginIndex > endIndex) {throw new StringIndexOutOfBoundsException(endIndex - beginIndex);}return ((beginIndex == 0) && (endIndex == count)) ? this :new String(offset + beginIndex, endIndex - beginIndex, value);
}public String concat(String str) {int otherLen = str.length();if (otherLen == 0) {return this;}char buf[] = new char[count + otherLen];getChars(0, count, buf, 0);str.getChars(0, otherLen, buf, count);return new String(0, count + otherLen, buf);
}public String replace(char oldChar, char newChar) {if (oldChar != newChar) {int len = count;int i = -1;char[] val = value; /* avoid getfield opcode */int off = offset;   /* avoid getfield opcode */while (++i < len) {if (val[off + i] == oldChar) {break;}}if (i < len) {char buf[] = new char[len];for (int j = 0 ; j < i ; j++) {buf[j] = val[off+j];}while (i < len) {char c = val[off + i];buf[i] = (c == oldChar) ? newChar : c;i++;}return new String(0, len, buf);}}return this;
}

String a = "chenssy";
String b = "chenssy";

String c = new String("chenssy");

/*** 采用字面值的方式赋值*/
public void test1(){String str1="aaa";String str2="aaa";System.out.println("===========test1============");System.out.println(str1==str2);//true 可以看出str1跟str2是指向同一个对象
}

/*** 采用new关键字新建一个字符串对象*/
public void test2(){String str3=new String("aaa");String str4=new String("aaa");System.out.println("===========test2============");System.out.println(str3==str4);//false 可以看出用new的方式是生成不同的对象
}

/*** 编译期确定*/
public void test3(){String s0="helloworld";String s1="helloworld";String s2="hello"+"world";System.out.println("===========test3============");System.out.println(s0==s1); //true 可以看出s0跟s1是指向同一个对象 System.out.println(s0==s2); //true 可以看出s0跟s2是指向同一个对象
}

/*** 编译期无法确定*/
public void test4(){String s0="helloworld"; String s1=new String("helloworld"); String s2="hello" + new String("world"); System.out.println("===========test4============");System.out.println( s0==s1 ); //false  System.out.println( s0==s2 ); //false System.out.println( s1==s2 ); //false
}

/*** 继续-编译期无法确定*/
public void test5(){String str1="abc";   String str2="def";   String str3=str1+str2;System.out.println("===========test5============");System.out.println(str3=="abcdef"); //false
}

/*** 编译期优化*/
public void test6(){String s0 = "a1"; String s1 = "a" + 1; System.out.println("===========test6============");System.out.println((s0 == s1)); //result = true  String s2 = "atrue"; String s3= "a" + "true"; System.out.println((s2 == s3)); //result = true  String s4 = "a3.4"; String s5 = "a" + 3.4; System.out.println((s4 == s5)); //result = true
}

/*** 编译期无法确定*/
public void test7(){String s0 = "ab"; String s1 = "b"; String s2 = "a" + s1; System.out.println("===========test7============");System.out.println((s0 == s2)); //result = false
}

/*** 比较字符串常量的“+”和字符串引用的“+”的区别*/
public void test8(){String test="javalanguagespecification";String str="java";String str1="language";String str2="specification";System.out.println("===========test8============");System.out.println(test == "java" + "language" + "specification");System.out.println(test == str + str1 + str2);
}

/*** 编译期确定*/
public void test9(){String s0 = "ab"; final String s1 = "b"; String s2 = "a" + s1;  System.out.println("===========test9============");System.out.println((s0 == s2)); //result = true
}

/*** 编译期无法确定*/
public void test10(){String s0 = "ab"; final String s1 = getS1(); String s2 = "a" + s1; System.out.println("===========test10============");System.out.println((s0 == s2)); //result = false }private static String getS1() {  return "b";
}

s = s.replace("World", "Universe");

public String(String original) {int size = original.count;char[] originalValue = original.value;char[] v;if (originalValue.length > size) {// The array representing the String is bigger than the new// String itself.  Perhaps this constructor is being called// in order to trim the baggage, so make a copy of the array.int off = original.offset;v = Arrays.copyOfRange(originalValue, off, off+size);} else {// The array representing the String is the same// size as the String, so no point in making a copy.v = originalValue;}this.offset = 0;this.count = size;this.value = v;}

/*** 关于String.intern()*/
public void test11(){String s0 = "kvill"; String s1 = new String("kvill"); String s2 = new String("kvill"); System.out.println("===========test11============");System.out.println( s0 == s1 ); //falseSystem.out.println( "**********" ); s1.intern(); //虽然执行了s1.intern(),但它的返回值没有赋给s1s2 = s2.intern(); //把常量池中"kvill"的引用赋给s2 System.out.println( s0 == s1); //flaseSystem.out.println( s0 == s1.intern() ); //true//说明s1.intern()返回的是常量池中"kvill"的引用System.out.println( s0 == s2 ); //true
}

/*** 关于equals和==*/
public void test12(){String s1="hello";String s2="hello";String s3=new String("hello");System.out.println("===========test12============");System.out.println( s1 == s2); //true,表示s1和s2指向同一对象，它们都指向常量池中的"hello"对象//flase,表示s1和s3的地址不同，即它们分别指向的是不同的对象,s1指向常量池中的地址，s3指向堆中的地址System.out.println( s1 == s3); System.out.println( s1.equals(s3)); //true,表示s1和s3所指向对象的内容相同
}

/*** String相关的+*/
public void test13(){String a = "aa";String b = "bb";String c = "xx" + "yy " + a + "zz" + "mm" + b;System.out.println("===========test13============");System.out.println(c);
}

public static main([Ljava/lang/String;)VL0LINENUMBER 5 L0LDC "aa"ASTORE 1L1LINENUMBER 6 L1LDC "bb"ASTORE 2L2LINENUMBER 7 L2NEW java/lang/StringBuilderDUPLDC "xxyy "INVOKESPECIAL java/lang/StringBuilder.<init> (Ljava/lang/String;)VALOAD 1INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;LDC "zz"INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;LDC "mm"INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;ALOAD 2INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;ASTORE 3L3LINENUMBER 8 L3GETSTATIC java/lang/System.out : Ljava/io/PrintStream;ALOAD 3INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/String;)VL4LINENUMBER 9 L4RETURNL5LOCALVARIABLE args [Ljava/lang/String; L0 L5 0LOCALVARIABLE a Ljava/lang/String; L1 L5 1LOCALVARIABLE b Ljava/lang/String; L2 L5 2LOCALVARIABLE c Ljava/lang/String; L3 L5 3MAXSTACK = 3MAXLOCALS = 4
}

String s = "a" + "b" + "c";
String s1  =  "a";
String s2  =  "b";
String s3  =  "c";
String s4  =  s1  +  s2  +  s3;

StringBuilder temp = new StringBuilder();
temp.append(a).append(b).append(c);
String s = temp.toString();

public class Test {public static void main(String args[]) {String s = null;for(int i = 0; i < 100; i++) {s += "a";}}
}

final StringBuffer a = new StringBuffer("111");
final StringBuffer b = new StringBuffer("222");
a=b;//此句编译不通过final StringBuffer a = new StringBuffer("111");
a.append("222");//编译通过

package com.spring.test;public class StringTest {public static void main(String[] args) {  /** * 情景一：字符串池 * JAVA虚拟机(JVM)中存在着一个字符串池，其中保存着很多String对象; * 并且可以被共享使用，因此它提高了效率。 * 由于String类是final的，它的值一经创建就不可改变。 * 字符串池由String类维护，我们可以调用intern()方法来访问字符串池。  */  String s1 = "abc";     //↑ 在字符串池创建了一个对象  String s2 = "abc";     //↑ 字符串pool已经存在对象“abc”(共享),所以创建0个对象，累计创建一个对象  System.out.println("s1 == s2 : "+(s1==s2));    //↑ true 指向同一个对象，  System.out.println("s1.equals(s2) : " + (s1.equals(s2)));    //↑ true  值相等  //↑------------------------------------------------------over  /** * 情景二：关于new String("") *  */  String s3 = new String("abc");  //↑ 创建了两个对象，一个存放在字符串池中，一个存在与堆区中；  //↑ 还有一个对象引用s3存放在栈中  String s4 = new String("abc");  //↑ 字符串池中已经存在“abc”对象，所以只在堆中创建了一个对象  System.out.println("s3 == s4 : "+(s3==s4));  //↑false   s3和s4栈区的地址不同，指向堆区的不同地址；  System.out.println("s3.equals(s4) : "+(s3.equals(s4)));  //↑true  s3和s4的值相同  System.out.println("s1 == s3 : "+(s1==s3));  //↑false 存放的地区多不同，一个栈区，一个堆区  System.out.println("s1.equals(s3) : "+(s1.equals(s3)));  //↑true  值相同  //↑------------------------------------------------------over  /** * 情景三：  * 由于常量的值在编译的时候就被确定(优化)了。 * 在这里，"ab"和"cd"都是常量，因此变量str3的值在编译时就可以确定。 * 这行代码编译后的效果等同于： String str3 = "abcd"; */  String str1 = "ab" + "cd";  //1个对象  String str11 = "abcd";   System.out.println("str1 = str11 : "+ (str1 == str11));  //↑------------------------------------------------------over  /** * 情景四：  * 局部变量str2,str3存储的是存储两个拘留字符串对象(intern字符串对象)的地址。 *  * 第三行代码原理(str2+str3)： * 运行期JVM首先会在堆中创建一个StringBuilder类， * 同时用str2指向的拘留字符串对象完成初始化， * 然后调用append方法完成对str3所指向的拘留字符串的合并， * 接着调用StringBuilder的toString()方法在堆中创建一个String对象， * 最后将刚生成的String对象的堆地址存放在局部变量str3中。 *  * 而str5存储的是字符串池中"abcd"所对应的拘留字符串对象的地址。 * str4与str5地址当然不一样了。 *  * 内存中实际上有五个字符串对象： *       三个拘留字符串对象、一个String对象和一个StringBuilder对象。 */  String str2 = "ab";  //1个对象  String str3 = "cd";  //1个对象                                         String str4 = str2+str3;                                        String str5 = "abcd";    System.out.println("str4 = str5 : " + (str4==str5)); // false  //↑------------------------------------------------------over  /** * 情景五： *  JAVA编译器对string + 基本类型/常量 是当成常量表达式直接求值来优化的。 *  运行期的两个string相加，会产生新的对象的，存储在堆(heap)中 */  String str6 = "b";  String str7 = "a" + str6;  String str67 = "ab";  System.out.println("str7 = str67 : "+ (str7 == str67));  //↑str6为变量，在运行期才会被解析。  final String str8 = "b";  String str9 = "a" + str8;  String str89 = "ab";  System.out.println("str9 = str89 : "+ (str9 == str89));  //↑str8为常量变量，编译期会被优化  //↑------------------------------------------------------over  }
}